Problem 20 - Analytical Chemistry

Question

Copper in saltwater near the discharge of a sewage treatment plant is determined by first separating and concentrating it by solvent extraction of its dithizone chelate at \(\mathrm{pH} 3\) into methylene chloride and then evaporating the solvent, ashing the chelate to destroy the organic portion, and titrating the copper with EDTA. Three \(1-\mathrm{L}\) portions of the sample are each extracted with \(25-\mathrm{mL}\) portions of methylene chloride, and the extracts are combined in a \(100-\mathrm{mL}\) volumetric flask and diluted to volume. A \(50-\mathrm{mL}\) aliquot is evaporated, ashed, and titrated. If the EDTA solution has a \(\mathrm{CaCO}_{3}\) titer of \(2.69 \mathrm{mg} / \mathrm{mL}\) and \(2.67 \mathrm{~mL}\) is required for titration of the copper, what is the concentration of copper in the seawater in parts per million?

Step-by-Step Solution

Answer

The copper concentration in the seawater is approximately 4.79 ppm.

Step 1: Calculate Total Mg of Cu

First, calculate the total milligrams of copper titrated using EDTA. Given that the EDTA solution has a titer of 2.69 mg/mL of \(\mathrm{CaCO}_3\), and 2.67 mL of this EDTA solution is required, the calculation is:\[2.67 \text{ mL} \times 2.69 \frac{\text{mg}}{\text{mL}} = 7.1823 \text{ mg of Cu}\]

Step 2: Calculate Concentration in the Flask

Since the 50 mL aliquot from the total 100 mL flask represents half (50% of the total solution), the amount of copper in the whole 100 mL solution is twice that in the 50 mL aliquot. So:\[7.1823 \times 2 = 14.3646 \text{ mg of Cu in 100 mL}\]

Step 3: Calculate Original Sample Copper Mass

To find out the amount of copper in the original 3000 mL (3 liters) of seawater, note that the 100 mL solution represents combined extracts from the 3 L. So:\[14.3646 \text{ mg/100 mL solution converted back to 3000 mL seawater}\]

Step 4: Calculate Copper Concentration in Seawater

To find the concentration in parts per million (ppm), recall that 1 ppm is equivalent to 1 mg of solute per 1 L of solvent. Therefore:\[\text{Concentration in ppm} = \frac{14.3646 mg}{3 L} = 4.7882 \text{ ppm}\]

Step 5: Conclusion

Therefore, the concentration of copper in the seawater sample near the sewage treatment plant is approximately 4.79 ppm.

Key Concepts

Solvent Extraction

Solvent extraction is a process used widely in analytical chemistry to separate and concentrate specific components from a mixture. It's like pulling a desired ingredient out of a complex soup. In the given exercise, solvent extraction is employed to isolate copper from seawater samples. Copper forms a complex with a substance called dithizone at a specific pH level, which helps in its separation. This complex is soluble in an organic solvent, methylene chloride, making it easy to extract from the aqueous phase.

During this process, multiple (25-mL) extractions of methylene chloride are performed to ensure maximum extraction of copper from the aqueous phase. Each extraction step enhances the separation, leading to a concentrated final product. After extraction, the solvent is evaporated, leaving behind the copper compound, ready for further analysis.

Chelating Agents

Chelating agents are compounds that can form multiple bonds with a single metal ion, creating a ring-like structure. Think of them as claws grabbing onto metals, which makes them very stable and unreactive. In this exercise, dithizone is used as a chelating agent to form a complex with copper ions in seawater.

Dithizone grabs onto copper ions, forming a chelate that helps isolate the copper during solvent extraction. This complexing step is crucial because it allows selective removal of copper from the seawater matrix, minimizing interference from other ions. Chelating agents like dithizone make the analytical process more specific and sensitive, ensuring that the copper is efficiently extracted and measured.

Titration

Titration is a method used to determine the concentration of a particular substance in a solution. It's like a balance game where you add a little bit of something known to precisely determine what you have. In this case, EDTA (a well-known chelating agent) is used in the titration to measure how much copper there is in the sample.

The copper complex, once isolated and treated to remove organic components, is titrated using EDTA. The titration process involves adding EDTA until it just fully reacts with the copper ions, which tells us exactly how much copper was there. The amount of EDTA used (2.67 mL in this exercise) is proportional to the amount of copper in the sample. This allows the calculation of copper concentration in the original seawater sample. Titration is crucial because it is a precise and accurate method for quantifying metal ions like copper.

Seawater Analysis

Seawater analysis involves a series of steps to detect and measure various compounds dissolved in seawater. Seawater is a complex mixture containing numerous ions and organic molecules, making its analysis challenging. In this exercise, we focus on measuring copper concentration, an essential aspect due to its environmental impact.

Precise analysis of seawater for trace metals requires methods like solvent extraction and chelating agents to concentrate and isolate the metal ions. After separating the copper, titration helps in quantifying its concentration. This specific example highlights the importance of monitoring heavy metals in marine environments, especially around sewage outfalls where contaminants might accumulate. Effective monitoring helps in understanding the ecological impact and managing pollution sources effectively. Detailed seawater analysis provides insights into the overall health of marine ecosystems.